1. Field of the Invention
The present invention relates to a beam irradiation apparatus and a beam irradiation method. Moreover, the present invention relates to a method for manufacturing a thin film transistor using the beam irradiation apparatus and the beam irradiation method.
2. Related Art
The research has been conducted on a thin film transistor having a poly-crystalline semiconductor film in its channel-forming region (hereinafter referred to as a poly-crystalline TFT) as a semiconductor element included in a display device, an integrated circuit, or the like. With the development of the display device and the integrated circuit, further enhancement of the characteristic of the poly-crystalline TFT is expected.
Consequently, in order to enhance the characteristic of the poly-crystalline TFT, crystallization of the semiconductor film by continuous wave (CW) laser light is examined. For example, there is a method for crystallizing an amorphous Si film in such a way that after the amorphous Si film is patterned to be linear or island-shaped over the glass substrate, an energy beam output from a CW laser oscillator continuously to time is irradiated to crystallize the amorphous Si film (refer to patent document 1). The patent document 1 describes that when the semiconductor thin film is patterned into linear or island-shaped in advance, the temperature of the glass substrate does not rise and therefore the crack or the like can be prevented. In addition, the patent document 1 also describes that a blocking plate with an opening provided is used to crystallize only the necessary part of the amorphous Si film selectively without giving damage to the glass substrate and without peeling the film with reference to FIG. 29 and FIG. 31.
In addition, a galvanometer mirror (refer to patent document 2) or a polygon mirror (refer to patent document 3) is used as means for scanning (also referred to as deflecting) the laser light (also described to as a laser beam). Since the galvanometer mirror and the polygon mirror can easily increase the scanning speed, it is possible to reduce the burden on the irradiation apparatus.    [Patent document 1]
Japanese Patent Application Laid-open No. 2003-86505    [Patent document 2]
Japanese Patent Application Laid-open No. 2003-86507    [Patent document 3]
Japanese Patent Application Laid-open No. 2003-45890
When the CW laser light as described above is irradiated to the substrate using the galvanometer mirror or the polygon mirror, a scanning width is limited. Therefore, it is necessary to repeat the scanning of the laser light for multiple times and there is the region where the scanning of the laser light stops. Such a region makes it difficult to perform the laser processing uniformly.
The galvanometer mirror and the like are originally built in the apparatus for marking. In such an application, the fact that the scanning of the laser light stops has not become a problem.
Moreover, the speed of the laser light scanned with the scanning means such as the galvanometer mirror or the polygon mirror is not constant in the center portion and in the end portion of the scanning width. For example, the laser light made to move back and forth in one direction by a galvanometer mirror decelerates toward a region where the direction of the back-and-forth movement changes (the point in which the galvanometer mirror stops) and its scanning speed becomes zero at last and then it is accelerated. In such a region where the scanning speed decelerates, accelerates, and becomes zero, the irradiation time becomes longer so that the object to be irradiated is irradiated with excessive energy. As a result, there is a risk that the amorphous semiconductor film is peeled and the like. It is concerned that when the film is peeled, the peeled film is spattered to the normal film, which results in the roughness of the normal film. Thus, it is a problem in the field of semiconductor that the scanning speed of laser light is not constant.
On the other hand, although the scanning speed does not become zero when the polygon mirror is employed, the scanning speed is not constant yet in the center portion and in the end portion of the scanning width. As a result, there is also a risk, as well as the galvanometer mirror, that the amorphous semiconductor film is peeled and the like.
As above, there is room for improvement in order to irradiate uniform CW laser light. Particularly when a large-sized substrate and mass production are taken into account, there are many points to be improved.